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Size effects in the magnetic anisotropy of embedded cobalt nanoparticles: from shape to surface.

Oyarzún S, Tamion A, Tournus F, Dupuis V, Hillenkamp M - Sci Rep (2015)

Bottom Line: Strong size-dependent variations of the magnetic anisotropy of embedded cobalt clusters are evidenced quantitatively by combining magnetic experiments and advanced data treatment.The obtained values are discussed in the frame of two theoretical models that demonstrate the decisive role of the shape in larger nanoparticles and the predominant role of the surface anisotropy in clusters below 3 nm diameter.

View Article: PubMed Central - PubMed

Affiliation: Institut Lumière Matière, UMR5306 Université Lyon 1-CNRS, Université de Lyon, 69622 Villeurbanne cedex, France.

ABSTRACT
Strong size-dependent variations of the magnetic anisotropy of embedded cobalt clusters are evidenced quantitatively by combining magnetic experiments and advanced data treatment. The obtained values are discussed in the frame of two theoretical models that demonstrate the decisive role of the shape in larger nanoparticles and the predominant role of the surface anisotropy in clusters below 3 nm diameter.

No MeSH data available.


Experimental results for the mean effective anisotropy constant as a function of the aspect ratio c/a (points).The horizontal bars for the experimental data correspond to the dispersions of the aspect ratio as estimated from TEM. The blue line shows the calculated shape anisotropy constants40.
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f3: Experimental results for the mean effective anisotropy constant as a function of the aspect ratio c/a (points).The horizontal bars for the experimental data correspond to the dispersions of the aspect ratio as estimated from TEM. The blue line shows the calculated shape anisotropy constants40.

Mentions: We calculated the demagnetizing energy for an ellipsoid from the demagnetizing factors (Nxx, Nyy and Nzz), computed by J.A. Osborn40 for a general ellipsoid in order to obtain the dependency of Keff as a function of the aspect ratio c/a (cf. Fig. 3). Here a prolate shape corresponding to a = b < c implies a uniaxial anisotropy whereas a general ellipsoid with a < b < c has a biaxial component to the anisotropy. We then convert the aspect ratio distribution as derived from TEM for the two samples with the larger particles into anisotropy distributions and compare them to those derived from the fits of the magnetic data. We find reasonable agreement between the distributions for the two larger sizes within our error bars of 10% (cf. figure 9, supplementary information). Note that the derived values of K2 for the largest clusters correspond to an aspect ratio c/a of ~1.3. We can thus conclude that for elongated nanoparticles the shape anisotropy is very important and can account by itself for the comparably high values derived in our experiment. This finding, however, cannot explain the increase by a factor of two when decreasing the size from 3.5 nm to 1.9 nm. For example a too small mean value of Keff ~ 60 kJ/m3 due to the shape is calculated for the aspect ratio c/a = 1.16 determined for the 2.7 nm sample.


Size effects in the magnetic anisotropy of embedded cobalt nanoparticles: from shape to surface.

Oyarzún S, Tamion A, Tournus F, Dupuis V, Hillenkamp M - Sci Rep (2015)

Experimental results for the mean effective anisotropy constant as a function of the aspect ratio c/a (points).The horizontal bars for the experimental data correspond to the dispersions of the aspect ratio as estimated from TEM. The blue line shows the calculated shape anisotropy constants40.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4593963&req=5

f3: Experimental results for the mean effective anisotropy constant as a function of the aspect ratio c/a (points).The horizontal bars for the experimental data correspond to the dispersions of the aspect ratio as estimated from TEM. The blue line shows the calculated shape anisotropy constants40.
Mentions: We calculated the demagnetizing energy for an ellipsoid from the demagnetizing factors (Nxx, Nyy and Nzz), computed by J.A. Osborn40 for a general ellipsoid in order to obtain the dependency of Keff as a function of the aspect ratio c/a (cf. Fig. 3). Here a prolate shape corresponding to a = b < c implies a uniaxial anisotropy whereas a general ellipsoid with a < b < c has a biaxial component to the anisotropy. We then convert the aspect ratio distribution as derived from TEM for the two samples with the larger particles into anisotropy distributions and compare them to those derived from the fits of the magnetic data. We find reasonable agreement between the distributions for the two larger sizes within our error bars of 10% (cf. figure 9, supplementary information). Note that the derived values of K2 for the largest clusters correspond to an aspect ratio c/a of ~1.3. We can thus conclude that for elongated nanoparticles the shape anisotropy is very important and can account by itself for the comparably high values derived in our experiment. This finding, however, cannot explain the increase by a factor of two when decreasing the size from 3.5 nm to 1.9 nm. For example a too small mean value of Keff ~ 60 kJ/m3 due to the shape is calculated for the aspect ratio c/a = 1.16 determined for the 2.7 nm sample.

Bottom Line: Strong size-dependent variations of the magnetic anisotropy of embedded cobalt clusters are evidenced quantitatively by combining magnetic experiments and advanced data treatment.The obtained values are discussed in the frame of two theoretical models that demonstrate the decisive role of the shape in larger nanoparticles and the predominant role of the surface anisotropy in clusters below 3 nm diameter.

View Article: PubMed Central - PubMed

Affiliation: Institut Lumière Matière, UMR5306 Université Lyon 1-CNRS, Université de Lyon, 69622 Villeurbanne cedex, France.

ABSTRACT
Strong size-dependent variations of the magnetic anisotropy of embedded cobalt clusters are evidenced quantitatively by combining magnetic experiments and advanced data treatment. The obtained values are discussed in the frame of two theoretical models that demonstrate the decisive role of the shape in larger nanoparticles and the predominant role of the surface anisotropy in clusters below 3 nm diameter.

No MeSH data available.